In recent years, attention has been paid to MIMO (Multi-Input/Multi-Output) communications as a technique that enables communications for large-capacity data such as image.
In MIMO communications, varying transmission data (sub-streams) is transmitted from multiple antennas at the transmitting side, and the receiving side separates the multiple transmission data mixed through the propagation path back to the original transmission data using propagation path estimation values (for example, see Unexamined Japanese Patent Publication No. 2002-44051 (FIG. 4)).
Actually, in MIMO communications, signals sent from transmitting apparatuses are received by the same or a greater number of antennas than the transmitting apparatuses, and propagation path characteristics between the antennas are estimated based on a pilot signal inserted in each signal received by the antennas. For example, when there are two transmitting antennas and two receiving antennas, the estimated propagation path characteristic, H, can be expressed in a matrix of 2 rows×2 columns. In MIMO communications, a transmission signal transmitted from each transmitting antenna is obtained based on the four components of the obtained propagation path characteristic H and a received signal obtained by each receiving antenna.
In this way, in MIMO communications, since the receiving side can separate the signals sent from the multiple transmitting antennas with the same timing and the same frequency into individual sub-streams, the amount of data proportional to the number of transmission antennas can be transmitted, thereby enabling high-speed and large-capacity communications.
In addition, in MIMO communications, since multiple data can be surely transmitted in parallel, transmission data per time is increased correspondingly. However, it is only when all the inter-antenna propagation paths have good characteristics that an increase in the amount of transmission data corresponding to the number of antennas can be expected, and, in actuality, there are few cases in which the propagation path characteristics are all good, and there are propagation paths with poor propagation path characteristics. In such case, at time of compensating other channel interference, an interference compensation error occurs in data transmitted through the propagation path due to noise and the like, and error rate characteristics upon demodulation of the received data decrease. At this time, if retransmission control is performed, the received data is determined as an error, and retransmission of data is repeated and the amount of overall transmission data practically decreases.